After PR-14958 was merged, I tested the latest development version of CPython on Arch Linux 5.2.3 and I can confirm that the "Settings" window no longer scales past the edge of the screen. Instead, the window size remains fixed and the scroll area dynamically changes to allow for preview of the enlarged font sample. To make sure it worked for all of the fonts, I scrolled the list of font faces with a size setting of 40 (as well as some other sizes, but not every possible combination). The window size remained the same and the "Font Sample" preview scaled dynamically with the scrollbar in every instance. 

This recent round of recent updates to the IDLE have added some fantastic QoL changes, nicely done taleinat and terry!